Mesobilirubinogen was prepared from the reduction of bilirubin crystals (Daiichi Kagaku Co. Ltd.) or extracted from the feces of a patient with hemolytic jaundice. Mesobiliviolins were obtained after oxidative procedures of mesobilirubinogen. Mesobiliviolin isomers were separated with thin layer chromatography by Stoll and identified with mesobiliviolin Ⅲa, Ⅸa and XⅢa respectively by mass spectrophotometry and nuclear magnetic resonance spectrometry. The identified mesobiliviolin isomers were studied spectrophotometrically. 1. Mesobiliviolin isomers prepared from the feces were of the same properties as that from bilirubin crystals in all examined items. 2. Mesobiliviolin isomers were found composed of three ones and identified with mesobiliviolin Ⅲa, Ⅸa and XⅢa by mass spectrometry and nuclear magnetic resonance spectrometry. 3. Mesobiliviolin Ⅲa dimethylester was reported instable by Stoll, but it was found stable during 8 hours if kept-in methanol in the dark at room temperature. 4. Absorption maxima of mesobiliviolin Ⅲa, Ⅸa and XⅢa dimethylester in methanol at pH 6.0 were 554.7nm, 555.0nm and 559.5nm, and in the case of nonesterified free form of them were 555.0nm, 555.6nm and 559.2nm respectively. 5. Absorption maxima of both their dimethyl ester and nonesterified free form of mesobiliviolin isomers migrated slightly to longer wave length in methanolic solution after acidification with hydrochloric acid at pH 1.0. 6. Conversion between methylesterified mesobiliviolin isomers did not occur under treatment of alkali or strong acid.